The present invention relates to a machining system that can handle most of the machine operations that a conventional milling machine could perform, but which is remotely controllable under water and in a radioactive, chemical or other toxic environment. The machining system is particularly useful for the repair and maintenance of the core barrel of a nuclear reactor.
As the various components within the core barrel of a nuclear reactor age, they require more frequent maintenance. Up to now, the approach taken by maintenance engineers has been to develop specialized tools on an ad hoc basis to perform a specific repair or maintenance operation. For example, specialized tools and procedures such as that illustrated in U.S. Pat. No. 4,720,898 were developed to replace worn out split pins in the guide tubes mounted above the fuel assemblies in the core of the reactor. Similarly, tools such as that illustrated in U.S. Pat. No. 4,292,133 were developed for replacing worn out control rod drive mechanisms in the upper core barrel of such reactors. The end result of this problem-solving philosophy has been the development of a relatively large number of expensive and specialized tools that must be used one-at-a-time to perform a series of maintenance operations on a single core barrel. The serial use of such specialized tools is, of course, time consuming as each tool must be set up, used and removed from the core barrel before the next tool may be used. Moreover, the specialized nature of each tool allows for little or no flexibility where a particular core barrel develops a unique problem. This is a serious limitation, as the utilities who manage such nuclear power facilities generally want all of the maintenance and repair needs of the core of a particular reactor to be dealt with during a single shutdown period. Hence, if an unexpected maintenance or repair need is discovered in the course of performing the planned repairs or maintenance operations, the maintenance personnel are often forced to immediately design, build and operate a new specialized tool so that all the necessary repairs will be completed within the same time period. The resulting time requirements for performing multiple maintenance tasks in a core barrel are not only expensive in monetary terms, as each day of down time of a nuclear reactor can cost a utility well over $1,000,000 in lost revenues a day, but further results in the exposure of the maintenance operators to significant amounts of potentially harmful radiation. While milling machines for performing general machining operations on large devices are, of course, known in the prior art, the Applicants are not aware of any such devices which could are compact enough to be practically and effectively used under water by remote control in the hostile, radioactive environment inside a core barrel.
Clearly, there is a need for a versatile machining apparatus capable of performing a multiplicity of different machining operations within the core barrel of a nuclear reactor in a safe, reliable and expeditous manner. Ideally, such a machining system should be rapidly installable within a core barrel and capable of performing machining operations within a few thousands of an inch. Finally, it would be desirable if the components of the machining system were easily decontaminable after use.